1. Field of the Invention
This invention relates to an electrophotographic apparatus, and more particularly to an improved transfer and electrostatic separating device.
2. Description of the Prior Art
Electrophotographic apparatuses are known which use an image bearing member such as an insulating drum or the like to form an image corresponding to an original. For example, in electrostatic copying machines, it has usually been practised to form a visible image corresponding to an original on a drum-like or belt-like photosensitive medium through a predetermined process, and to transfer the visible image to a transfer medium to thereby obtain a final copy. In this case, as the method of transferring the image, there is a method whereby corona discharge opposite in polarity to the charge retained by fine coloring particles (hereinafter referred to as toner) forming the visible image on the photosensitive medium is applied to the back side of the transfer medium to thereby cause the toner to adhere to the transfer medium to electrostatic attraction, or a method which uses, instead of corona discharge, a roller having a bias voltage applied thereto, to transfer the image on the basis of a principle similar to what has been described above.
When the transfer medium is to be separated from the photosensitive medium, a charge opposite in polarity to the toner is present on the back side of the transfer medium and under the influence of such charge, the transfer medium adheres to the photosensitive medium. Therefore, means is required for overcoming such adhesion force to mechanically separate the transfer medium from the photosensitive medium or for negating the adhesion force itself.
As an example of the former means, there is a method whereby a separating belt member is interposed between the photosensitive medium and the transfer medium and when the transfer medium has passed the transfer area, the belt guides and separates the transfer medium from the photosensitive medium. This method ensures reliable separation of the transfer medium while, on the other hand, it has the disadvantages that a non-image bearing area is created on the final copy and that the separating member contacts the photosensitive medium and is thus liable to damage the surface of the photosensitive medium.
As an example of the latter means, there is a so-called electrostatic separating method whereby the charge on the back side of the transfer medium is negated as by corona discharge to thereby permit the transfer medium to separate from the photosensitive medium due to its own rigidity and weight (U.S. Pat. Nos. 3,575,502 and 3,998,536). As compared with the former method of the contact type, this latter method of the non-contact type has numerous merits including a merit that no blank area is created on the final copy and a merit that the photosensitive medium is not damaged. On the other hand, however, this latter method has a demerit that the adhesion force of the transfer medium to the photosensitive medium resulting from the charge of the back side thereof depends on various factors such as the resistivity and thickness of the transfer medium, the surface potential of the photosensitive medium, atmospheric conditions, etc. and therefore, the reliability of separation is not always high.
In view of this, there has heretofore been proposed a method of changing the separation discharging conditions in accordance with the type of the transfer medium or a method whereby the difference in surface potential of the photosensitive medium corresponding to the black ground portion and the white ground portion of an original (hereinafter referred to as the black original and the white original or the image bearing portion and the non-image bearing portion) is reduced before the transfer charging to prevent the adhesion force of the transfer medium to the photosensitive medium from being greatly varied in accordance with the black original or the white original.
The latter method will hereinafter be described. In that portion of the photosensitive medium surface where the surface potential of the photosensitive medium when contacted by the transfer medium corresponds to the black original (the image bearing portion), for example, the photosensitive medium surface is charged to +600 volts while that corresponding to the white original (the non-image bearing portion) is 0 volt. Next, when corona discharge opposite in polarity to toner, i.e. plus in this case, is effected through the transfer medium, the amount of corona discharge current which is determined by the surface potential of the photosensitive medium and the voltage applied to the corona discharge wire becomes greater for the white original than for the black original. Also, even when the same amount of positive charge exists on the back side of the transfer medium, the static electricity acts so as to cause the transfer medium to adhere more strongly to the photosensitive medium surface having a surface potential of minus tendency and therefore, a transfer medium for the black original can be readily separated but separation of a transfer medium for the white original is difficult under the separation discharging conditions for the black original.
To eliminate such difficulty, the step of converging the surface potential of the photosensitive medium to a predetermined value before the image transfer may be incorporated. More specifically, there has been proposed a method of reducing this surface potential difference by using a charger (U.S. Pat. No. 3,357,400) or a method of obtaining the same result by applying a bias light to the photosensitive medium. It is known that the difference in separation performance resulting from the difference between the black and white of an original can be improved by these methods, but to achieve such purpose, means such as a charger or a lamp must additionally be provided and this would lead to bulkiness or complication of the apparatus. Also, completely eliminating the surface potential difference resulting from the black original and the white original by the charging before the image transfer would become more difficult for a high speed copying process and moreover, the elimination of the potential difference by application of the light of a lamp or the like would result in insufficient discharging at the potential of the black original in which toner is present.